These filter devices are readily available on the market in a plurality of designs and versions. Among their functions is to filter dirt in fluids, such as hydraulic oil, out of these fluids. Hydraulic oil is fouled during installation and when the respective hydraulic system is started up. In addition to this initial fouling, fouling during operation can occur, for example by penetration of dirt at the hydraulic tank due to inadequate tank ventilation, pipe penetrations, piston rod seals, and the like. To the extent fouling within the fluid stored in the hydraulic tank occurs in hydraulic systems of machines such as earth moving machines, excavators or the like, it can be advantageous to implement filtration directly in the area of the hydraulic tank, for example by attaching the filter device directly to the tank. The hydraulic oil removed from the tank is delivered directly to a filter element to filter out dirt which filter element is held in the filter housing. The fluid which has been cleaned in this way then returns again to the tank by the filter housing. Here add-on solutions are known in which the filter device cleans only the contents of the tank. Solutions are also conceivable in which the filter device delivers the correspondingly filtered and cleaned fluid to the hydraulic circuit of the machine, in order from there to return to the tank the fluid fouled with solid particles in the pertinent circulation in the hydraulic circuit.
Where the filter device filters only the contents of the tank, fluid is removed from the tank, filtered and then returned again to the tank. To avoid interruptions in the operation of the respective hydraulic system, the contents of the fluid means, especially in the form of a hydraulic tank, is advantageously blocked off relative to the filter housing to replace a fouled and used filter element by a new one, or for changing this filter element to completely dismount the filter housing with the used filter element from the fluid means in the form of the tank. After appropriate maintenance of the unit of the filter housing with the filter element, especially by replacement of the used filter element by a new one, this unit can be re-connected to the fluid means for use. It is also possible to connect a new unit of the filter housing with the unused filter element to the fluid means, while in the other dismounted unit the filter element is changed. For this reason, in the known solutions the pipes must be separated from each other in a complicated manner, sealed, and by complex rotary slide valve parts, the fluid connection between the fluid means (tank) and the filter housing with the filter element must be separated and later re-connected. The rotary slide valve parts, as a component of a connecting means between the filter housing and the fluid means, are not only expensive to produce and complicated to maintain, but also entail the danger that correspondingly large amounts of fluid will emerge from the fluid means as a leaking oil flow. This leakage leads to fouling problems in the vicinity. These solutions are also prone to failure in operation. In this regard, they are less reliable, since an operator often has problems at the assumed position of the rotary slide valve parts in recognizing whether they are in their blocking or in their open position.